In the manufacture of printed circuit boards, frequently literally thousands of small holes must be drilled into each printed circuit board. In volume production of the circuit boards, the drilling of holes is accomplished by computer controlled automatic drilling machines in which the printed circuit boards are usually mounted on a worktable which is movable in a horizontal X-Y plane. A sheet of back-up material is typically mounted on top of the worktable, and beneath the printed circuit boards, so that the drill tool does not drill into the worktable upon passing through the underside of the printed circuit board.
Usually, the worktable moves horizontally to be positioned a spindle having a drill tool mounted therein, so that the holes may be drilled at appropriate drilling locations. Drilling of the circuit boards is accomplished by advancing each drill spindle downward through a vertical drilling stroke. The present invention relates to optimally controlling the termination of the downward stroke of the spindle.
A vital concern in the field of printed circuit board drilling machines is the speed with which a machine can drill holes. This is usually referred to as the productivity or "throughput" of a drilling machine. Although the time it takes to drill any single hole is relatively small, each circuit board drilled usually requires drilling thousands of holes, for example as many as 20,000 or more holes per board. Consequently, any small variance in the time to drill a single hole has a greatly multiplied effect and is very significant in the long term.
Another concern is that of minimizing drilling of the back-up material. When holes are drilled completely through a printed circuit board, the tool must penetrate the back-up material to some degree in order for the drill hole to be uniform throughout the entire depth of the circuit board. However, it is desirable to minimize the amount of penetration into the back-up material because the time required for such drilling lengthens the drill stroke and also causes wear of the drill. Further, circuit boards are commonly drilled in stacks. The deeper the hole is to be drilled, the longer the drill tool must be. In turn, the longer a drill tool is relative to its diameter, the more easily the tool will break. By decreasing the depth of tool penetration into the back-up material, shorter tools can be used for a given diameter hole, thus decreasing the likelihood of tool breakage.
Prior art through drilling techniques have not been satisfactory in satisfying these performance criteria. Prior art machines include a sensor to determine the vertical position of the drill spindle, and in turn the tip of the drill tool which extends a nominal distance from the spindle. One approach has been to simply drill to a preselected vertical position which is sufficiently deep to insure that the workpiece is fully drilled through. However, to accommodate several variables, the preselected depth must be so far beneath the top surface of the backup material to result in substantial amounts of unnecessary drilling of back-up. For example, the back-up material is not perfectly planar, and thus the position of its top surface can only be estimated. Further, due to drill wear, the precise position of the tip of the drill tool changes. During use the drill tools gradually become worn and thus they become measurably shorter over time. Prior to drilling into a workpiece, therefore, the vertical or Z-axis position of the tip of a worn drill tool is often higher than its expected position, namely, the Z-axis position of the tip of a new, unworn drill.
Using this prior art technique in a multi-spindle drilling machine, all the drill tools penetrate beyond the workpiece and into the back-up material below the workpiece so that even the worn drill tools will be ensured of drilling completely through the workpiece. As a result, unworn and only nominally worn drill tools drill unnecessarily deep into the back-up material, causing lost time during unnecessary drill movement. Additionally, drilling into the back-up material causes further wear of the drill tool, and causes loosened particles of the back-up material to be carried up into and embedded into the walls of the hole drilled in the workpiece, thereby decreasing the quality of the hole.
One solution to the above problem is to frequently replace all the drill tools on the drilling machine with new, unworn drill tools. This solution, however, is unnecessarily expensive and wastes drill tool which could otherwise be sharpened and reused.
Another approach has been to use a technique designed for drilling holes only partially through a workpiece, called "depth control drilling." In depth control drilling, the variance or "offset" in vertical position of a worn drill tool from the nominal position of a new drill tool is determined before drilling begins, so that the precise vertical position of the tip of the drill tool is known. When the drill stroke is commenced, the depth of the hole is measured from the top of the workpiece. This reference position is sensed when a pressure foot, which is attached to the drill spindle, engages the top of the workpiece. When the desired depth is reached, the drill stroke is terminated. This technique is adapted to through drilling by treating the depth of the hole to be drilled as equal to the thickness of the workpiece or stack of workpieces desired to be through drilled.
The shortcoming of using the depth control technique for through drilling is that sensing the vertical position of the top surface of the workpiece, from which the depth of the hole is measured, can be inexact. For example, inaccuracies arise due to debris on top of the workpieces. Likewise, an assumption must be made regarding the thickness of the workpiece which is being drilled in order to determine how deep the hole must be in order to drill completely through the workpiece. This assumption may be inaccurate since waviness in the workpieces can result in varying workpiece thicknesses depending on the location at which the hole is being drilled. These inaccuracies are multiplied when several printed circuit boards are stacked for through drilling. Also, debris between stacked workpieces can cause variation in workpiece thickness.
Thus a need exists for an accurate through drilling technique which insures that holes are drilled completely through the workpiece, while also minimizing unnecessary drilling of the back-up material.